There are two types of color cameras that use an imaging element in their image pick-up part: one is a multi-chip CCD which uses a plurality of image elements and the other is a single-chip CCD which uses a single imaging element. A the single-chip CCD camera has an arrangement of color separation filters which corresponds to pixels of the CCD. The arrangement is selected to have predetermined spectral characteristics. In the single-chip CCD, one pixel outputs only one signal, but a plurality of chrominance signals and a luminance signal corresponding to one pixel can be obtained by operating a plurality of signals from the adjacent plural pixels. Inevitably, the single-chip CCD causes a two-dimensional sampling frequency of each chrominance signal to become low, so that false signals or chrominance moires appear in the luminance signals or the chrominance signals due to a high-pass spectrum of an image.
A conventional imaging element uses a PD(Photodiode)-Mixing-System which sequentially reads two signals from two pixels on two lines that adjoin each other in the vertical direction simultaneously. The two signals are added. A sequential chrominance system is an example of the conventional pick-up system using the imaging element. FIG. 11 shows an arrangement of a minimum unit of color separation filters. Such a unit is repeatedly arrayed in the sequential chrominance system. FIG.13 shows a method for reading the signals from the pixels of the CCD according to the PD-Mixing-System. In FIG. 13, the numeral 130 shows the arrangement of the color separation filters and the letters of "Mg", "G", "Ye" and "Cy" in the boxes designate the spectral characteristics of the filters. For example, a filter 131 designated by "Mg" can pass only magenta colored light. On the other hand, the letters of "Mg", "G", "Ye" and "Cy" in both sides of the arrangement of the color separation filters 130 designate the output signals from the pixels designated by the same letters. In the PD-Mixing-System shown in FIG. 13, Ye(yellow)+Mg(magenta), Cy(cyanogen)+G(green) are serially output from even-number lines of the first field (left side of the color separation filters 130). Hereupon, Ye(yellow)=r(red)+g(green), M(magenta)=r(red)+b(blue) and Cy(cyanogen)=g(green)+b(blue). Thus, a signal {2r(red)-g(green)} can be obtained by operating (Ye+Me)-(Cy+G). On odd lines, Cy+Mg and Ye+G are output point-sequentially, so that a signal {2b(blue)-g(green)} can be obtained by operating (Cy+Mg)-(Ye+G). On the second field, Mg+Cy and G+Ye are output point-sequentially on even lines, so that a signal (2b-g) can be obtained by operating (Mg+Cy)-(G+Ye). On odd lines, Mg+Ye and G+Cy are output point-sequentially. Therefore, a signal (2r-g) can be obtained by operating (Mg+Ye)-(G+Cy).
In the sequential chrominance system mentioned above, color carriers exist in the horizontal direction of two-dimensional frequency, and a color separation is obtained through an operation of output signals from the pixels adjoined in the horizontal direction, that is, through an one-dimensional operation. As a result, a color moire appears in the horizontal direction of a picture. Furthermore, only one chrominance signal can be obtained from the output signals from the pixels arrayed on one horizontal line. Furthermore, a color moire appears in the vertical direction of two-dimensional frequency since a frequency of a two-dimensional sampling of R(red) and B(blue) lights is low in the vertical direction. However, the color moires in horizontal and vertical directions of two-dimensional frequency stand out more due to a visual characteristic of a human being. Thus, this is one of main causes of a poor picture quality. These moires are caused by the high-frequency spectrum of two-dimensional frequency of an incident picture. Therefore, they were reduced to some degree by using an optical low-pass filter such as a crystal optical filter in order to reduce the high-frequency spectrum of two-dimensional frequency. The reduction of the two-dimensional high-frequency spectrum of the incident picture by using the optical low-pass filter, however, was only possible up to a certain limit because this method was closely connected with poor resolution.
FIG. 12 shows the above-mentioned carriers when the arrangement of the color separation filters shown in FIG. 11 is used. Px and Py indicate distances between two adjoined pixels, or the width of the pixels in the horizontal and vertical directions respectively. The abscissa of FIG. 12 shows the sampling frequency of specific chrominance signals in horizontal direction, and the ordinate of FIG. 12 shows the sampling frequency of the specific chrominance signals in the vertical direction. The carriers can be obtained by Fourier transformation of the sampling pattern of respective chrominance signals. The directions of the arrows in FIG. 12 designate the phases of the carriers. In view of the vertical line 1/2Px, the directions of arrows Ye and Cy or Mg and G are opposite to each other. Such differences of the directions of the carriers cause the appearance of the luminance moires in the horizontal direction, which causes poor resolution in the horizontal direction.
A luminance signal can be obtained by adding a plurality of chrominance signals from the adjoined pixels corresponding to several different color separation filters. It is known to minimize the luminance moire when an additive ratio of obtaining the luminance signal by adding each chrominance signal is determined to be the inverse of a sensitivity ratio of each chrominance signal. However, since the sensitivity of each chrominance signal depends on spectral characteristics of incident light, the additive ratio must also be changed according to the spectral characteristics of the incident light.
Generally, an image characteristically has strong color correlation in a localized picture area. A method to reduce the moires by applying this characteristic is described in Publication Gazette of Unexamined Japanese Patent Application Hei 2-166988 titled "Solid Color Camera" or described in Television Gakkaishi Vol.46, No. 9, pp. 1153-1160 (1992) etc.
This system is briefly explained in the following. For example, when the sequential chrominance system using the PD-Mixing image element and the color separation filters shown in FIG. 11 is applied, either chrominance signals, Mg+Ye and G+Cy or Mg+Cy and G+Ye, are output sequentially. If there is no color change (monochromatic and even), a ratio of Mg+Ye:G+Cy:Mg+Cy:G+Ye is constant. Thus, on a line outputting Mg+Ye and G+Cy, G+Cy is interpolated by an operation (G+Cy)'=(G+Cy).sub.L /(Mg+Ye).sub.L .times.(Mg+Ye) against a position of pixel which outputs Mg+Ye, and Mg+Ye is interpolated by an operation (Mg+Ye)'=(Mg+Ye).sub.L /(G+Cy).sub.L .times.(G+Cy) against a position of pixel which outputs G+Cy. (Mg+Ye).sub.L and (G+Cy).sub.L are low-pass spectrums of Mg+Ye and G+Cy, respectively, which are obtained by passing signals Mg+Ye and G+Cy through a low-pass-filter. In other words, the luminance moire is reduced by the interpolation operation according to the ratio of chrominance signals in which a color moire was reduced by the low-pass-filter.
When using the arrangement of the color separation filter shown in FIG. 11 which renders the color carriers shown in FIG. 12 in the horizontal direction, the low-pass filter for reducing the color moire must have a zero point on a vertical line of 1/4Px in FIG. 12. In order to reduce the moires in the vicinity of the vertical line of 1/2Px in FIG. 12, it is necessary to use a narrow-band low-pass filter. However, while the interpolation operation is conducted as mentioned above, if an object shows a point of drastic color change, false signals appear because of an error between the color signal having a limited band and an actual color. The narrower the band of the low pass filter, the greater the error at the color boundary. Thus, the problem of false signals occurring from this error develops in a broad range, and therefore, it is difficult to reduce the luminance moire in the vicinity of the vertical line of 1/2Px.